The present technology relates to a cable comprising a cable layer on polypropylene basis with low dielectric loss. Furthermore, the present technology is related to a process for the manufacture of such a cable.
Manufacture of a low attenuation and recyclable cable with high stiffness and high temperature resistance is highly desired.
In certain applications, the communication cables must guarantee a good operating mode. This means that the dielectric loss at certain frequencies needs to be below a certain threshold limit, i.e. be as low as possible. This will enable the cable manufacturer to control the overall losses taking place in the cable. Typically, these losses increase with an increasing frequency. The loss rests upon two main causes: 1. conductor loss and 2. dielectric loss (material). The latter is directly dependent on the frequency whilst the conductor loss is dependent of the square root of the frequency. Thus the higher the frequency of operation, the more important the dielectric losses become. This is typically the case for higher category data cables and radio frequency cables.
Today, polyethylene is used as the material of choice for the insulation of these cables due to the ease of processing and the beneficial electrical properties. The insulation is typically foamed in order to obtain even more beneficial dielectric properties and to ensure dimensional stability. However, in order to assure good operating properties at the required operating temperature, there is a need to crosslink polyethylene either by peroxides or silanes. As a result of crosslinking, there are less recycling options and there is limited processing speed due to dependency on the crosslinking speed.
Thus it is searched for a potential candidate, which can replace the commercial polyethylene on the market, i.e. there is the need to provide cables with low dielectric loss and that do not show the drawbacks of the known cables comprising layers on polyethylene basis.
Polypropylene is in principle considered as such a potential candidate in the field of the communication application area. Polypropylene has the following advantages over polyethylene under particular circumstances:                Lower dielectric constant, allowing downsizing of the cable dimension or decrease of the foaming degree;        Increased hardness;        Decreased dielectric loss at higher frequencies.        
However, there is a shared opinion in this technical field that the above stated beneficial properties can be only achieved with highly ‘clean’ polypropylene materials, i.e. free of the presence of species (e.g. catalyst residues) that can affect the dielectric loss in a negative way.
Accordingly the polypropylene which is nowadays available and fulfils the appreciated high standards, must be after its manufacture troublesome washed to remove any species affecting negatively the dielectric properties.
In addition, of course, any replacement material, i.e. any polypropylene which is suitable to replace polyethylene in this technical field of communication cables, must still have good mechanical and thermal properties enabling failure-free long-run operation of the cable. Furthermore, any improvement in processability should not be achieved on the expense of mechanical properties and any improved balance of processability and mechanical properties should still result in a material of low dielectric loss.
EP 0 893 802 A1 discloses cable coating layers comprising a mixture of a crystalline propylene homopolymer or copolymer and a copolymer of ethylene with at least one alpha-olefin. For the preparation of both polymeric components, a metallocene catalyst can be used. The polymers have acceptable thermal stability. However the dielectric loss of the cable is rather high and additionally the polymers are not suitable to be foamed.
DD 203 915 describes a foam from a composition containing LDPE which shows a low dielectric loss (<2×10−4). However, these products lack temperature resistance and stiffness.
JP 2006 022 276 describes a foam from HDPE which shows a dielectric loss tangent value (tan δ) less than 1.3×10−4 at 2.45 GHz. However the temperature resistance of polyethylenes is inadequate because of the low melting temperature. Also, the material does not provide sufficient stiffness.
JP 2001 354 814 describes a polypropylene multiphase composition with one component with a dielectric loss of at least tan δ>3×10−3. Moreover the materials as disclosed therein cannot be foamed.
EP 1 429 346 A1 describes a polypropylene composition containing a clean polypropylene and a strain hardening polypropylene. However clean polypropylene materials are difficult to make and more importantly, they cannot be foamed unless blended with high melt strength polypropylene (HMS-PP). If blended, the dielectric loss deteriorates dramatically.